This invention relates to an iron based alloy having an oriented grain structure which is characterized by a cube-on-edge orientation (described in Miller indices as (110) [001] grain orientation), and having a primary recrystallized and normal grain growth microstructure. Such magnetic materials are useful, for example, as core materials in power and distribution transformers.
The operating inductions of a large portion of today's transformers are limited by the saturation value of the magnetic sheet material which forms the core. In extensive use today is an iron-based alloy containing nominally 3.25 percent silicon (all composition percentages herein are in weight percent) which is processed in order to obtain cube-on-edge or (110) [001] grain orientation in the final product. A well-known example of this type of steel called type M-5. These 3.25 percent silicon steels have the final grain orientation developed by means of a secondary recrystallized microstructure. This microstructure is attained during the final box annealing in which preferentially oriented grains grow at the expense of non-preferentially oriented grains with the result that the alloy usually has an extremely large grain structure size in which the diameter usually greatly exceeds the thickness of the sheet material. Obtaining such large grains in a secondarily recrystallized microstructure requires a long time, high temperature anneal for the development of the orientation. The extensive anneal is generally also required for the reduction of residual sulfur content. Sulfur contents in excess of about 100 ppm in the finished product adversely affect the magnetic characteristics exhibited by the silicon-iron alloy.
In addition to the costly long time, high temperature anneal, the addition of 3.25 percent silicon to pure iron, while effective and generally desirable for improving the volume resistivity, nevertheless lowers the saturation value in most commercially produced 3.25% silicon containing iron alloys to generally less than 20,300 gauss. Thus, there is the obvious trade-off as the improved resistivity (which lowers core losses of the material) is obtained at the expense of saturation value (significantly lower than the saturation value of about 21,500 gauss of commercially pure iron). Moreover, since commercial iron has substantially higher core losses and substantially higher coercive force values than silicon steel, it was prudent to balance the overall magnetic characteristics and the best balance heretofore obtained was that of the 3.25-percent-silicon iron alloy which exhibited the cube-on-edge orientation.
A primary recrystallized alloy alternative to the generally used commercial alloy is described in U.S. Pat No. 3,849,212, issued Nov. 19, 1974, and the associated primary recrystallization method is described in U.S. Pat. No. 3,892,605, issued July 1, 1975 (both to Thornburg). These relate to an iron base alloy made from an ingot containing up to about 0.03 percent carbon, up to 1 percent manganese, from about 0.3 to about 4 percent of at least one of the volume resistivity improving elements selected from the group consisting of up to about 2 percent silicon, up to 2 percent chromium, and up to about 3 percent cobalt. The balance of the alloy is essentially iron with incidental impurities. Thornburg's method utilizes processing by hot working and either a two- or three-stage cold rolling operation, with the final cold rolling stage working effecting only a moderate (50-75 percent) reduction in the cross sectional area of the material being processed. These prior patents deal in relatively broad ranges of composition and do not recognize the criticality between constituents.